The overall basic design of a self-lapping type control valve, for which the present invention is adapted, is known in the prior art and is described in Japanese Pat. Nos. 57-36185 and 59-19866. The structure described in these patents is illustrated in FIG. 3. With regard to this item, the diaphragm element 2, which is retained by the piston 1 at its inner periphery, is retained at its outer periphery on the main valve body 3; and by means of the diaphragm element 2 and the piston 1, the internal space of the main valve body 3 is divided into the control chamber (a), which is in communication with the brake line BP and the constant pressure chamber (b), which is in communication with the constant pressure air reservoir CR. The constant pressure chamber (b) is connected to the control chamber (a) via the check valve CHV, which establishes one-way flow of fluid pressure from control chamber (a) to constant pressure chamber (b) and cuts off fluid pressure flow in the reverse direction. The main valve body 3 is also equipped with a valve mechanism in which the compressed air route, which is in communication with the brake cylinder BC, is opened or closed by the movement of the piston 1 as a function of the pressure difference between the control chamber (a) and the constant pressure chamber (b). Also provided is a balance piston 51 having a diaphragm element 2 which forms, on one side, an output chamber (c) subject to the brake cylinder pressure; and on the other side, an exhaust chamber (d) having an exhaust outlet which opens to the atmosphere. A supply chamber is provided which is connected with a supply reservoir SR, via a supply valve 54, that is normally engaged with a valve seat 53 by a spring 52. Fixed on the balance piston 51 is an exhaust valve rod 55, one opened end of which is adjacent the above-mentioned supply valve 54, the middle portion of which is open to the exhaust chamber (d), and the other end portion of which penetrates the wall between the exhaust chamber (d) and the control chamber (a) (so that it can slide freely and in an airtight manner), being connected with the above-mentioned piston 1. In operation, when the brake command, namely, the pressure effective in brake line BP, is reduced, only the pressure in the control chamber (a) decreases, because of the operation of the check valve CHV, while the constant pressure chamber (b) is kept at a constant pressure; and as a result of this pressure difference between the control chamber (a) and the constant pressure chamber (b), the piston 1 moves upwardly (as viewed in the drawing) and the exhaust valve rod 55 and the balance piston 51, which are connected to the piston 1, rise. Accordingly, the exhaust valve rod 55 engages and pushes up the supply valve 54 against the force of the spring 52, to separate it from the valve seat 53.
In this manner, the open end of exhaust valve rod 55 is closed while concurrently, the compressed air passage, formed by the supply chamber (e) and the output chamber (c), is opened, and the compressed air in supply reservoir SR flows into the brake cylinder BC. Accordingly, the pressure in the BC increases, and the brake is applied. When the BC pressure reaches a certain pressure corresponding to the pressure difference described above, the balance piston 51 is forced down, and the supply valve 54 with which the exhaust valve rod 55 is already engaged, seats on the valve seat 53; then the supply valve 54, the valve seat 53, and the exhaust rod 55 coact to maintain the brake cylinder pressure constant. This status is the brake-holding status, and the BC pressure is controlled as a function of the pressure-difference described above.
When pressure in brake line BP is increased by the brake-release command, the control chamber (a) is pressurized and the piston 1 moves down, and the exhaust valve rod 55 separates from the valve 54. Therefore, the compressed air in the BC is released into the atmosphere, passing through the output chamber (c), the passage in the exhaust valve rod 55, and the exhaust chamber (d), and the brake is released.
FIG. 4 shows the installation of the piston 1, the diaphragm element 2, and the main valve body 3 in the self-lapping type control valve, and one example in which the check valve is located in the piston 1. The installation of the piston 1, the diaphragm element 2, and the main valve body 3 is as shown in the prior art arrangement of FIG. 3.
In the mating groove 11 which is formed on the outer circumference of piston 1, a projection 21, which is narrower than the mating groove 11 described above, is formed on the inner periphery of the diaphragm element 2, the diameter of which is slightly smaller than the inside diameter of the mating groove 11, so that delta L (the distance between the outer surface end of the piston in the control chamber (a) and the inner wall of the main valve body 3) and delta L' (the distance between the outer surface end of the piston in the constant pressure chamber (b) and the inner wall of the valve main body 3) are minimized.
In this manner, the installation and removal of the diaphragm element 2 (corresponding to the piston 1) is facilitated, since the inner peripheral area of the diaphragm element 2 is fixed to the piston 1 without bolts, etc.
Following is an explanation of the check valve CHV, which is illustrated in FIG. 4. In this check valve CHV on the cylindrical hub 12, which is formed at the center of the piston 1 and which has a bottom surface, the throttle hole 13 (the inner portion of which is in communication with the constant pressure chamber (b)) is cut; and on the lower end of the exhaust valve rod 55 which is fitted into the cylindrical hub 12, there is a rubber valve 6 which faces the throttle hole 13 described above. A spring 7, inside the cylindrical hub 12, acts against a flange 57 of the exhaust valve rod 55 having a connecting passage 56 which connects the interior of the cylindrical hub 12 and the control chamber (a). The upper portion of the flange 57 engages a retaining ring 8, which is fitted onto the cylindrical hub 12 to prevent the exhaust valve rod 55 and the cylindrical hub 12 from slipping down. Accordingly, when the pressure is decreased in control chamber (a) in response to the brake command, the piston 1 and its cylindrical hub 12 rise with the diaphragm element 2 against the force of the return spring 4, so that the spring 7 is compressed and the surrounding rim area of the throttle hole 13 is pushed onto the rubber valve 6.
Therefore, fluid pressure communication between the constant pressure chamber (b) and the control chamber (a) is interrupted. This condition is also maintained in the brake-holding state. In response to the brake-release command, the piston 1 and the diaphragm element 2 descend under the force of the increased pressure in the control chamber (a); and as a result of the force of the spring 7, the piston 1 and its cylindrical hub 12 descend in relation to the exhaust valve rod 55; the rubber valve 6 opens the connecting passage 13, and the constant pressure chamber (b) is in communication with the control chamber (a) via the throttle hole 13, the inner space of the cylindrical hub 12, and the connecting passage 56.
Therefore, a lack of pressure, which sometimes occurs in the constant pressure chamber (b), is filled with the compressed air which is supplied from the BP. However, the arrangement illustrated in FIG. 4 requires numerous extra parts, such as, the rubber valve 6, the spring 7, the retaining ring 8, etc., and the design is quite inconvenient.
Also, since it requires the formation of the cylindrical hub 12 on the piston 1 and/or the formation of the flange 57 on the exhaust valve rod 55, the construction of the valve becomes more complicated, which is another disadvantage. Therefore, the arrangement illustrated in FIG. 5 has been suggested.
In the FIG. 5 arrangement, there is a ledge 9 on the wall surface of the control chamber (a) facing the diaphragm element 2. An opening end 101 of the passage 10, which connects, at the other end, with the constant pressure chamber (b), passes through ledge 9 and opens into chamber (a). The bottom end of the exhaust valve rod 55 is fixed on the piston 1 by means of the retaining ring 8.
Accordingly, when fluid pressure in control chamber (a) is decreased in response to the braking command, the piston 1 and the diaphragm element 2 rise against the force of the return spring 4; and as shown in FIGS. 6 and 7, the diaphragm 2 contacts the ledge wall surface 92, which is between the end rim 91 of the ledge 9 and the opening end 101 of the passage 10, so that the opening end 101 is closed and the constant pressure chamber (b) is cut off from the control chamber (a).
Also, when the control chamber (a) is pressurized to obtain a brake release, the piston 1 and the diaphragm element 2 descend, and the diaphragm element 2 separates from the ledge wall surface 92. As shown in FIG. 5, the opening end 101 of the passage 10 opens, and the constant pressure chamber (b) is placed in communication with the control chamber (a) via the passage 10.
FIG. 6 illustrates the case in which the pressure difference between the control chamber (a) and the constant pressure chamber (b) is large; namely, the difference between BP and CR is great (high braking status). FIG. 7 shows the case in which the pressure difference between the control chamber (a) and the constant pressure chamber (b) is small; namely, the difference between BP and CR is low (low braking status).
Let us now consider the sealing ability of the diaphragm element 2 contacting the ledge wall surface 92 in the high-braking status and the low-braking status. In the low-braking status illustrated in FIG. 7, the contact pressure of the ledge wall surface 92 and the diaphragm element 2 is relatively low, so that the seal is weak; therefore, a pressure bleed, due to the backflow of compressed air from CR to BP, tends to occur, thus causing the brake to be inadvertently released. This problem sometimes occurs at the transition between the brake-release status (illustrated in FIG. 5) and the low-braking status (illustrated in FIG. 7). However, it has been discovered that it occurs most often during so-called stepwise or graduated release, in which the BP is pressurized gradually, starting from the high-braking status (illustrated in FIG. 6) and it attempts to maintain the low-braking status (illustrated in FIG. 7). This is because the former is the so-called closing condition, in which the diaphragm element 2 is pulled up with the rising piston (the diaphragm element 2 is rising in the direction to close the opening end 101); but the latter is the so-called opening condition (the condition in which the diaphragm element 2 is descending in the direction to open the opening end 101) in which the diaphragm element 2 loosens from the status where the diaphragm element 2 is strongly pulled up and is stretched to a maximum in the high-braking status. In the low-braking or high-braking status described above, when the pressure in CR is P.sub.CR and the pressure in BP is P.sub.BP, the sealing force F can be determined by Equation (1): EQU F=(P.sub.CR -P.sub.BP).times.A.times.1/2 (1)
where A is the surface area of the ring-shaped portion of the diaphragm element 2 which corresponds to L', which is the distance between the rim end 91 of the ledge 9 and the contact portion of which the diaphragm element 2 contacts the mating groove 11 of the piston 1 (called the annular surface). However, the force of each spring is eliminated here. In Equation (1), the pressure difference P.sub.CR -P.sub.BP can be varied, depending on the brake control status; therefore, the cause of the pressure bleed is that the annular surface A is small.
It is possible to enlarge the annular surface A by reducing the outer diameter of piston 1, to make delta L larger. As described in Japanese Pat. No. 58-15735, the diaphragm element 2, which fits into the mating groove 11 of the piston 1, tends to slip out; and at the same time, the diaphragm element 2 must be reinforced, which is not desirable.